Thousands of people in the United States have joined nationwide rallies to demand federal action in support of civil rights, one week after a Florida jury acquitted a Hispanic neighborhood watchman in the shooting death of an unarmed black teenager.

The National Action Network, led by American civil rights leader Rev. Al Sharpton, organized Saturday's vigils in honor of the teenager, Trayvon Martin, outside government buildings in more than 100 cities around the country.

They were the latest in a series of national protests against the July 13 acquittal of the former neighborhood watchman George Zimmerman, who was found not guilty of second degree murder and manslaughter in the February 2012 incident in which he fatally shot Martin in the chest during an altercation. The jury accepted Zimmerman's claim that he was acting in self-defense.

The verdict angered many Americans who believe Martin's killing was racially motivated. Race was not discussed in court during the trial.

Saturday's main rally was in New York City, where hundreds of people gathered to see Martin's mother Sybrina Fulton, Sharpton and pop superstar couple Beyonce and Jay-Z. Addressing the crowd, Fulton said "today it was my son, tomorrow it might be yours." She also vowed to fight for change in the country to ensure that her son's death is not in vain, saying "I am going to work for your children as well."

Fulton's former husband Tracy Martin also led a rally of hundreds of people in Miami, Florida.

Sharpton is pressing the U.S. Department of Justice to launch a civil right prosecution of Zimmerman on suspicion of targeting Martin because of the teenager's race. Some legal experts say it would be difficult for federal prosecutors to prove the allegation.

President Barack Obama paid tribute to Martin in a surprise appearance before White House reporters on Friday, saying the teenager "could have been my son ... could have been me 35 years ago."

The nation's first black president urged people to see the Martin case from the perspective of African-Americans, saying he and others have experienced society perceiving them as threatening because of their race.

Obama also said he is looking into ways to examine state and local laws to see if they encourage confrontations like the one in Florida.